1. Field of the Invention
The present invention relates to a device for automatically controlling the idle speed of an automotive engine in response to changes in engine operating conditions, such as the engine cooling water temperature, the ambient air temperature and so forth, to maintain the idle speed at a predetermined set level regardless of such changes in the engine operating conditions. More particularly, the invention is concerned with an electronically controllable actuator suitable for use in such engine idle speed control devices.
2. Description of the Prior Art
Japanese Patent Pre-Examination Publication No. 116966/1981 discloses an engine idle speed control device operative to control the rate of air flow through a bypass air passage extending in bypassing relationship to a throttle valve in an intake pipe of the engine. More specifically, this device employs a pair of bypass air metering valves arranged such that vacuum forces act on the valves in opposite directions to eliminate any influence by the vacuum force which would otherwise adversely affect the air metering characteristics in the case where only one such valve is used.
In the known engine idle speed control device, the condition of balance between the forces acting on the air metering valves was liable to be changed due to a change in the bypass air flow rate, depending on factors such as the shape of the bypass air passage, the shapes of the air metering valves and so forth. Thus, it was difficult to obtain a valve displacement which is exactly proportional to a supply of electric current to a solenoid section of the device.